This invention relates to apparatus for observing chemiluminescent reactions and more particularly to apparatus employing a single photodetector and a plurality of chambers wherein chemiluminescent reactions may occur.
Because of efforts being undertaken to reduce atmospheric pollution, reliable methods are needed for monitoring the level of various individual noxious gases in both the ambient atmosphere and various effluent sources, such as vehicle exhaust and the like. The detection of the presence of pollutants in sub-part-per-million levels by the observation of chemiluminescent reaction is particularly attractive because the method can be adapted to be continuous and because long path length observation is not required, as in absorption spectroscopy. A chemiluminescent reaction occurs where a primary reactant such as nitric oxide (NO) or carbon monoxide (CO), which are common pollutants, engage in a highly exothermic reaction with certain second reactants, or reagents, such as atomic oxygen (O) or ozone (O.sub.3) to emit radiant energy usually in the infrared region. The mechanism and kinetics of the chemiluminescent reaction of NO with O.sub.3 have been described by P. N. Clough and B. A. Thrush, Trans. Faraday Soc. 63, 915 (1967). Sensitive detectors can be calibrated to respond to the chemiluminescent emission in proportion to the concentration of the primary reactants in the sample. Methods have been devised to measure the concentration of substances which are not directly measureable by chemiluminescent reaction but which bear an ascertainable relation to substances which do. For example, the concentration of NO.sub.x, which is the mixture of NO and NO.sub.2, as well as the concentration of NO and NO.sub.2 are of interest in applications such as the measurement of vehicle emission and the like. However, NO.sub.2 does not readily react with ozone or the like in a chemiluminescent reaction. NO.sub.2 may be converted to NO by appropriate catalytic or reactive methods, permitting the measurement of NO.sub.x by the chemiluminescent measurement of the equivalent amount of NO. The concentration of NO.sub.2 may be determined thereafter by comparing the measured concentration of NO with measured concentration of NO.sub.x.
In some applications it is desirable to observe the chemiluminescent reactions of interest under reaction conditions of very low pressure. However, systems have been devised which operate satisfactorily near ambient pressure, thus eliminating the requirement of a cumbersome and expensive vacuum system for maintaining low pressures. It remains necessary to employ a reaction volume at least large enough to observe and detect measurable chemiluminescence, the extent of which is measurably decreased as pressure is increased. This decrease in measurable chemiluminescence is commonly denoted as the quenching effect.
Other limitations include the effects of short-term ambient noise on the sensitivity of the detection system. Noise effects can be reduced by optical chopping techniques known to the art and by signal integration over relatively long sampling periods (greater than a few seconds).
Some of the sampling and integration techniques known to the art include the comparison of data obtained during substantially different time intervals. If, however, the concentrations of the substances to be measured vary substantially from one period to the next, the data obtained may be meaningless. One technique which overcomes this disadvantage is the simultaneous measurement of several samples, which permits the simultaneous comparison of data. According to the aforementioned detection technique, more than one detector is employed, typically one detector per reaction chamber. Since detectors tend to be bulky and expensive and since detectors having substantially identical operating characteristics are difficult to provide, thereby admitting to a margin of error in response among nonidentical detectors, it is advantageous to employ a single detector to perform all measurements.
It is, therefore, an object of the present invention to provide apparatus for observing chemiluminescence in a sample.
It is a further object to provide apparatus for analyzing a variety of samples substantially simultaneously.
It is a further object to provide apparatus for chemiluminescent analysis having a minimum number of detectors.
It is a further object to provide apparatus having reaction chambers wherein observable chemiluminescent reaction may occur near ambient pressure.
It is further an object of this invention to minimize the volume of reaction chambers wherein observable ambient pressure chemiluminescent reactions may occur in order to maximize the number of reaction chambers which may be viewed by a single photodetector.
It is further an object of this invention to minimize the size and expense of a detection system comprising reaction chambers and a photodetector.
It is further an object of the invention to provide apparatus for continuous and substantially simultaneous analysis of at least two constituents in a sample.
It is a further object of this invention to minimize the effects of ambient noise and component instability in a chemiluminescent analyzer.
Other objects and features will be in part apparent and in part pointed out hereinafter.